Maize adaptation to temperate climate: relationship between population structure and polymorphism in the Dwarf8 gene

Abstract

To investigate the genetic basis of maize adaptation to temperate climate, collections of 375 inbred lines and 275 landraces, representative of American and European diversity, were evaluated for flowering time under short- and long-day conditions. The inbred line collection was genotyped for 55 genomewide simple sequence repeat (SSR) markers. Comparison of inbred line population structure with that of landraces, as determined with 24 SSR loci, underlined strong effects of both historical and modern selection on population structure and a clear relationship with geographical origins. The late tropical groups and the early "Northern Flint" group from the northern United States and northern Europe exhibited different flowering times. Both collections were genotyped for a 6-bp insertion/deletion in the Dwarf8 (D8idp) gene, previously reported to be potentially involved in flowering time variation in a 102 American inbred panel. Among-group D8idp differentiation was much higher than that for any SSR marker, suggesting diversifying selection. Correcting for population structure, D8idp was associated with flowering time under long-day conditions, the deletion allele showing an average earlier flowering of 29 degree days for inbreds and 145 degree days for landraces. Additionally, the deletion allele occurred at a high frequency (>80%) in Northern Flint while being almost absent (<5%) in tropical materials. Altogether, these results indicate that Dwarf8 could be involved in maize climatic adaptation through diversifying selection for flowering time.

Figure 1.

Models for population structure at three steps of maize selection history: landraces (seven groups), first-cycle inbreds (five groups), and whole inbred panel (five groups). Groups for each panel are represented by colors as indicated at the bottom. For the inbred panels, each inbred line is represented by a vertical line divided into colored segments, the length of which indicates the proportion of the genome attributed to the different groups. For the landrace panel, each population is represented by the mean proportions estimated for the five inbred lines simulated to represent it. Solid arrows stand for filiation relationship between clusters and have been established on the basis of either Structure assignments (in the joint study of landraces and first-cycle inbreds) or genetic distances between groups of inbred line panels (Table 3). Dashed arrows indicate lower contributions [less than three inbred lines with a high genome proportion (g_ij > 0.80) attributed to a group obtained in the Structure joint analysis of landraces and first-cycle inbred lines].

Figure 2.

Geographical distribution of D8-deletion frequency among landraces.